feat(server): implement management endpoints and redis sync

Enable full resource management via API and support data plane
synchronization.

- Add CRUD handlers for Providers, Models, and Keys using DTOs
- Implement LogWriter service for asynchronous, batched audit logging
- Update SyncService to snapshot full configuration state to Redis
- Register new API routes and initialize background services
- Add configuration options for logging performance tuning

internal/model/log.go

@@ -0,0 +1,22 @@
+package model
+
+import "gorm.io/gorm"
+
+// LogRecord stores lightweight metadata for auditing. Avoids full payload unless audit is triggered.
+type LogRecord struct {
+	gorm.Model
+	Group        string `json:"group"`
+	KeyID        uint   `json:"key_id"`
+	ModelName    string `json:"model"`
+	StatusCode   int    `json:"status_code"`
+	LatencyMs    int64  `json:"latency_ms"`
+	TokensIn     int64  `json:"tokens_in"`
+	TokensOut    int64  `json:"tokens_out"`
+	ErrorMessage string `json:"error_message"`
+	ClientIP     string `json:"client_ip"`
+	RequestSize  int64  `json:"request_size"`
+	ResponseSize int64  `json:"response_size"`
+	AuditReason  string `json:"audit_reason"`
+	RequestBody  string `json:"request_body"`  // optional, only when audit triggered
+	ResponseBody string `json:"response_body"` // optional, only when audit triggered
+}